1. Field of the Invention
The present invention relates to a bar type vibration motor for generating vibration by rotation of an eccentric weight, and more particularly, to a bar type vibration motor capable of improving a body structure for supporting a rotary shaft fixed to a eccentric weight, a coupling structure of a stationary member and the rotary shaft, and a contacting structure of a commutator and brushes in order to facilitate fabrication, more securely support the rotary shaft, and miniaturize itself.
2. Description of the Related Art
As portable communication instruments generally used at present, mobile phones have various signal-generators to transmit various signals to users.
Therefore, when messages or calls are received, the signal-generators generate sound, light or vibration so that users informed of the incoming of messages or calls.
The signal-generators are generally adopted as sound generators, illumination devices and vibrators.
On the other hand, the vibrators have various vibration motors as vibration sources, in which the vibration motors are usually classified into flat type vibration motors and bar type vibration motors according to their configurations.
The flat type vibration motors are also called coin type vibration motors because they are shaped as thin coins, and the bar type vibration motors are also called cylinder type vibration motors because they have cylindrical configurations.
Both the flat type vibration motors and the bar type vibration motors are operated on the basis of the electromagnetic induction regardless of their configurations.
The electromagnetic induction is a phenomenon in which electromagnetic force is generated across the magnetic field, when current is flown through conductors placed perpendicular to the magnetic field.
The vibration motors convert electric energy into mechanical energy on the basis of the electromagnetic induction and generate vibration from the mechanical energy.
FIG. 1a illustrates a conventional bar type vibration motor that will be described hereinafter.
As shown in FIG. 1a, the conventional bar type vibration motor 100 includes a stator unit 110, a rotor unit 130 and a power supply unit 150, in which the stator unit 110 including a body 111 and a magnet 116 will be explained first.
As shown in FIG. 1b, the body 111 includes a housing 112 and a yoke 114. The housing 112 is shaped as a pipe having opened both ends, and the yoke 114 includes a hollow cylindrical yoke body 114b, a bearing insert groove 114a formed at the front end of the yoke body 114b and a flange 114c formed on the periphery of the bearing insert groove 114a. 
The body 111 has a double-pipe structure placing the yoke body 114b in the housing 112 by pressing and welding the flange 114c of the yoke 114 to one end of the housing 112.
As shown in FIG. 1a, the body 111 formed by fixing the yoke 114 to the housing 112 and a magnet 116 is attached on the outer surface of the yoke body 114b of the body 111.
Next, the rotor unit 130 will be explained.
As shown in FIG. 1a, the rotor unit 130 includes an eccentric weight 131, a rotary shaft 132, a commutator 134 and an armature 136. The rotary shaft 132 is fixed to the eccentric weight 131 having an eccentric center of gravity at one end thereof, and a stationary member 138 at the other end thereof.
The armature 136 is disposed around the rotary shaft 132, fixed to the periphery of the stationary member 138 parallel with the rotary shaft 132. The armature 136 has a structure coiled by a wire (not shown) or includes coils (not shown).
On the other hand, the commutator 134 having several separate segments is attached on the side of the stationary member 138.
The commutator 134 is made of conductive materials, and electrically connected with the armature 136.
In this case, the stationary member 138 has a cylindrical projection 138a extruded from one side of the stationary member 138.
Therefore, the commutator 134 having the separate segments is attached on the side of the stationary member 138 to surround the projection 138a and the side of the stationary member 138.
As above mentioned, the rotor unit 130 is rotatably mounted on the stator unit 110.
In other words, as shown in FIG. 1a, the rotary shaft 132 is inserted into the yoke body 114b, and rotatably supported at one end thereof by a first bearing 102 inserted into the bearing insert groove 114a and at the other end thereof by a second bearing 104 inserted into the rear end of the yoke body 114b. 
In this case, the armature 136 is spaced apart from the magnet 116.
Next, the power supply unit 150 will be explained.
The power supply unit 150 includes a fixing cap 152 and a pair of brushes 154 mounted in the fixing cap 152.
The brushes 154 are touched with the commutator 134 surrounding the periphery of the projection 138a by coupling the fixing cap 152 to the other end of the housing 112.
At this time, the brushes 154 are provided with supply voltage through lead wires (not shown) connected with the brushes 154.
The voltage applied to the brushes 154 as above is in turn supplied to the commutator 134 touched with the brushes 154.
Therefore, when the wire or the coils (not shown) of the armature 136 is energized by the voltage to the commutator 134, electromagnetic force is generated through the interaction between the armature 136 and the magnet 116 attached on the outer surface of the yoke body 114b. 
When the electromagnetic force is applied to the armature 136, as the rotary shaft 132 is rotated, vibration is generated by rotating the eccentric weight 131 fixed to the one end of the rotary shaft 132.
However, the conventional bar type vibration motor has following problems.
As shown in FIG. 1b, since the body 111 is obtained by presseing the yoke body 114b into the housing 112, and then welding then together, it is difficult to apply the body 111 to a miniaturized vibration motor.
In other words, as the bar type vibration motor is miniaturized, the thickness of the housing 112 and the york 114 also get thin.
Therefore, when the yoke 114 is pressed into the housing 112 first, the flange 114c of the yoke 114 is bent or deformed under the pressure.
Also, when the pressed portion between the housing 112 and the yoke 114 is welded after the pressing, there is a problem that the pressed portion is thermally deformed due to the thinness of the housing 112.
An integral body 111′ shown in FIGS. 2a and 2b was proposed to solve the above problem.
In other words, the proposed body 111′ has a double-pipe structure in which a support tube 111a′ is formed integrally in the body 111′ and a bearing insert groove 111b′ is formed at one end of the support tube 111a′. 
However, a conventional bar type vibration motor using the integral body 111′ has following problems.
When impact is applied to a mobile phone mounted with a conventional bar type vibration motor using the integral body 111′, the impact is transferred to first and second bearings 102 and 104 supporting the rotary shaft 132.
In this case, the first bearing 102 disposed more adjacent to the eccentric weight 131 for supporting the rotary shaft 132 is more frequently deformed than the second bearing 104.
Also, when the vibration is generated by the rotation of the rotary shaft 132, there is a problem that the first bearing 102 is worn away earlier than the second bearing 104.
Because larger load is applied to the first bearing 102 disposed more adjacent to the eccentric weight 131, the degree of the deformation or abrasion occurred on the first 102 is different from that of the second bearing 104.
On the other hand, when the rotary shaft 132 rotates, the abrasion and deformation of the bearings prevents the rotary shaft 132 from rotating smoothly causing undesirable noise.
Therefore, there was a problem that the expected life span of a bar type vibration motor was shortened.
As a solution to the above problem, there was proposed an approach for increasing the length of the first bearing, on which bigger load is exerted to reduce the deformation or abrasion.
That is, this approach increases the depth of the bearing insert groove 111b′ formed in one end of the body 12 and inserts a longer bearing or several bearings into the bearing insert groove 111b′, in order to reduce damage or abrasion of the bearings brought by impact.
But, if the depth of the bearing insert groove 111b′ is increased to prolong the length of the bearing inserted into the bearing insert groove 111b′ as above, the length of the magnet 116 is to be reduced in proportion to the reduction of a space in the body 111′. This brings an another problem of degrading the performance of the vibration motor by the reduction of an area for forming a magnetic field.
Therefore, because the body 111′ can be formed integrally, the body 111′ can be manufactured without deformation occurred by pressing or welding. But the expected life span of the bar type vibration motor was shortened due to the abrasion of a bearing by the eccentric weight 131 or the deformation of a bearing supporting the rotary shaft under the external impact.
Also, as shown in FIGS. 1a and 2a, because the rotary shaft 132 is fixedly inserted into the stationary member 138, the thickness of the stationary member 138 should be increased to improve the axial coupling force between the rotary shaft 132 and the stationary member 138.
On the other hand, it is difficult to miniaturize the vibration motor, because a projection 138a is formed on the side of the stationary member 138 to contact the brushes 154 with the commutator 134.
And, because the commutator 134 is divided into several segments, sparks are generated between the commutator 134 and the brushes 154, when the brushes 154 touch the segments from one to an other.
Unfortunately, the sparks occuring as above damage the commutator 134 and the brushes 154.